1. Field of the Invention
The present invention relates to process and apparatus for hardening mechanical parts of a ring-shaped sheet such as a bearing washer for a needle roller thrust bearing, or other holed flat parts which are formed by punching a metal sheet, without being accompanied by any deformation.
2. Related Art
The annular flat parts of chromium-molybdenum steel having a thickness of about 1 mm and a diameter of about 80 mm, as used as a bearing washer of a needle thrust bearing, are so conveyed in a carburizing and hardening furnace while being loosely laid on a mesh belt conveyor, for example, according to the prior art that they are heated to about 880.degree. C. in the carburizing atmosphere and then quenched in an oil bath.
According to the above-specified hardening process, however, the flat parts are deformed or strained, as exemplified by a warp, so that they have to be corrected to restore their original shape after the hardening operation. As a result, the number of steps increases to cause a problem that the production cost rises. With a serious deformation, the correction cannot be sufficient thereby to leave the strain, thus causing a problem that the percentage of defective products is high. Moreover, the flat parts have to be delivered into the inside of the furnace such that they are arranged without any overlap on the mesh belt. For a massive working of the flat parts, therefore, there arises another problem that the furnace has to be large-sized to require a wide space.